feat: add OperationFailedError with actionable hints

The Kanboard API returns only true/false for many operations without
explaining why they failed. Added OperationFailedError type that
includes operation details and hints about possible causes.

Updated MoveTaskPosition and MoveTaskToProject to use this new error
type, providing users with actionable debugging information instead
of generic "failed to move task" messages.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

errors.go

@@ -82,6 +82,26 @@ func (e *APIError) Error() string {
 	return fmt.Sprintf("Kanboard API error (code %d): %s", e.Code, e.Message)
 }
 
+// OperationFailedError represents an API operation that returned false without details.
+// The Kanboard API often returns only true/false without explaining why an operation failed.
+type OperationFailedError struct {
+	Operation string
+	Hints     []string
+}
+
+// Error implements the error interface.
+func (e *OperationFailedError) Error() string {
+	msg := fmt.Sprintf("%s: operation failed", e.Operation)
+	if len(e.Hints) > 0 {
+		msg += " (possible causes: " + e.Hints[0]
+		for _, hint := range e.Hints[1:] {
+			msg += ", " + hint
+		}
+		msg += ")"
+	}
+	return msg
+}
+
 // IsNotFound returns true if the error indicates a resource was not found.
 func IsNotFound(err error) bool {
 	return errors.Is(err, ErrNotFound) ||
@@ -102,3 +122,9 @@ func IsAPIError(err error) bool {
 	var apiErr *APIError
 	return errors.As(err, &apiErr)
 }
+
+// IsOperationFailed returns true if the error is an OperationFailedError.
+func IsOperationFailed(err error) bool {
+	var opErr *OperationFailedError
+	return errors.As(err, &opErr)
+}